Optimally, license plates have an overall similarity of styling or appearance that enables rapid recognition of license plates issued by various licensing authorities and that inhibits counterfeiting. At the same time, license plates should provide a distinct, individualized and unique identifying code or image for each vehicle, state, or driver. To that end, many licensing authorities offer “vanity” license plates. Such plates allow the driver to select an attractive or meaningful design or message that will be printed on their license plate. The production of such “vanity” license plates results in each state offering numerous different license plate designs to its constituents.
Meanwhile, automated enforcement systems, including, for example, electronic toll systems, red light miming systems, speed enforcement systems, and access control systems, are becoming more prevalent. Many embodiments of such systems rely on an accurate reading of a vehicle's license plate, which is often performed by an automated license plate recognition (ALPR) system. However, obtaining an accurate reading of a vehicle's license plate is becoming increasingly difficult due to the wide variety of license plates now on the roads.
Automatic Vehicle Identification (AVI) is a term applied to the detection and recognition of a vehicle by an electronic system. Exemplary uses for AVI include, for example, automatic tolling, traffic law enforcement, searching for vehicles associated with crimes, and facility access control. Ideal AVI systems are universal (i.e., they are able to uniquely identify all vehicles with 100% accuracy). The two main types of AVI systems in use today are (1) systems using RFID technology to read an RFID tag attached to a vehicle and (2) systems using a camera or optical device and a computer to read a machine-readable code attached to a vehicle.
One advantage of RFID systems is their high accuracy, which is achieved by virtue of error detection and correction information contained on the RFID tag. Using well known mathematical techniques (cyclic redundancy check, or CRC, for example), the probability that a read is accurate (or the inverse) can be determined. However, RFID systems have some disadvantages, including that not all vehicles include RFID tags. Also, existing unpowered “passive” RFID tag readers may have difficulty pinpointing the exact location of an object. Rather, they simply report the presence or absence of a tag in their field of sensitivity. Moreover, many RFID tag readers only operate at short range, function poorly in the presence of metal, and are blocked by interference when many tagged objects are present. Some of these problems can be overcome by using active RFID technology or similar methods. However, these techniques require expensive, power-consuming electronics and batteries, and they still may not determine position accurately when attached to dense or metallic objects.
Machine vision systems (often called Automated License Plate Readers or ALPR systems) use a machine or device to read a machine-readable code attached to a vehicle. In many embodiments, the machine-readable code is attached to, printed on, or adjacent to a license plate. One advantage of ALPR systems is that they are can be used almost universally, since almost all areas of the world require that vehicles have license plates with visually identifiable information thereon. However, the task of recognizing visual tags can be complicated. For example, the read accuracy from an ALPR system is largely dependent on the quality of the captured image as assessed by the reader. Existing systems have difficulty distinguishing tags from complex backgrounds and handling variable lighting. Further, the accuracy of ALPR systems suffers when license plates are obscured or dirty.
Some exemplary ALPR systems include a bar code (or other machine-readable portion) containing an identification code which will provide information about the vehicle. Typically, the bar code on a license plate includes inventory control information (i.e., a small bar code not intended to be read by the ALPR). Some publications (e.g., European Patent Publication No. 0416742 and U.S. Pat. No. 6,832,728) discuss including one or more of owner information, serial numbers, vehicle type, vehicle weight, plate number, state, plate type, and county on a machine-readable portion of a license plate.